12 Jun Can I get performance issues with a link that is configured over water?
Yes, this is common if one of the following conditions hold:
- The Fresnel zone is not clear (see figure below where the waves are close to the Fresnel zoner boundary). Even if the zone is not exactly on the boundary you could experience problems. The issues get worse the more opening angle and stronger side lobes you have on your antenna. The reason is that you get multipath copies that bounce off the water and arrive at the receiver with misaligned phase. This results in lower received signal strength or it might just corrupt the packets. You will see this as low RSSI (compared to what is expected) and/or high levels of retransmissions, PHY errors and checksum errors.
Remedy: Use radio nodes with external antenna ports and connect high gain parabola antennas on each side. This gives you higher RSSI margin and reduces the main/side lobes which gives less multipath propagation.
- One (or both) of the antennae is moving due to that it is placed on a boat or something similar. In these cases, it could be wise to set up two or more clients on different heights and configure the system in a multipoint fashion to increase the stability.
Tip 1: Tidal waves don’t just follow a 24-hour cycle. They also vary in height based on the position of the moon and sun so the actual height of a high wave is different each day during a month. By using Repeatits free cloud service, you can easily monitor the trends for your links and correlate the link performance and RSSI figures with the tidal wave information.
Tip 2: Unless you really need bandwidth, use a Trinity 323 configured at a 20MHz spectrum bandwidth (rather than 40MHz) and enable the Packet Heal functionality but setting the “Always buffer” parameter to Yes and change the “Reordering level” parameter to High in the Trinity radio’s web GUI configuration tab. This will improve TCP level stability if you have packet errors as additional error correction is added on top of the normal radio layer error correction.